The field of this invention of that of crankshaft driven pumps which are used to produce pressurized fluid, typically at relatively high pressures. A conventional crankshaft driven pump has a crankshaft, connecting rod, and piston very much like an automotive engine. It will typically have an intake valve for each cylinder to draw fluid into the cylinder area on the “down stroke” of the piston, or the portion of the stroke when the volume of the cylinder area is increasing. On the returning “up stroke” or the portion of the stroke when the volume of the cylinder area is decreasing, the fluids will be forced out the cylinder through another valve. This can happen on a cylinder or any number of cylinders. A triplex pump is one with three cylinders and is a very common combination in oilfield operations.
As the piston moves up and down due to the rotation of the crankshaft, the up and the down position of the piston are typically very well defined. This means that the pump will pump a very predictable volume of fluid, or will have a positive displacement for each rotation of the crankshaft.
When the pump is driven by a single speed electric motor, the total volume pumped will simply be the positive displacement for each rotation of the crankshaft times the number of revolutions per minute.
There are occasions when it is desirable to have different flow rates from the pump. This is conventionally achieved by getting a variable speed motor or by having intermediate components which change the single speed of a motor to a variable speed for the pump. The variable speed motor always seems like a simple solution, but especially in high horsepower applications and applications in explosive environments the motors become very expensive.
The intermediate components to achieve variable flow also tend to be complex. One solution is to install a gear box, but this is complex and can require that the system be stopped to change gears. Alternately a variable displacement hydraulic pump and a hydraulic motor can be installed between the electric motor and the triplex pump. This is space consuming, expensive and prone to need maintenance.